The present invention concerns a thermoplastic-coated roll for a paper/board machine, a method for manufacture of the roll, a composition of thermoplastic coating for the roll, a method of calendering by means of thermoplastic-coated rolls in accordance with the invention, and paper/board manufactured by means of such rolls.
In a paper/board machine, currently polymer-coated rolls are employed in a number of applications. As examples of such rolls can be mentioned press rolls, suction rolls, soft rolls for a supercalender, and so-called soft-calender rolls. So far, the coatings have been mainly based on polymers and in particular on the use of thermosetting-plastic based resins, such as polyurethane or epoxy or unsaturated polyester. From the point of view of technology of manufacture, these polymers have been suitable for coating of large rolls; among other things, their casting in moulds or rotation casting is possible, even though problems of work hygiene have arisen from vapours of volatile organic compounds, such as styrene. Further manufacturing techniques that have been used include, among other things, extrusion, spraying, winding of fibre, winding of band, centrifugal casting, and use of various impregnated mats.
As an example of a thermosetting-resin coated roll should be mentioned the patent publication EP 321,561, in which a calender roll is described whose metal core has been coated with a resin layer consisting of a thermosetting plastic, such as polyurethane, polyisocyanurate, cross-linked polyesteramide, or epoxy resin.
The use of thermosetting plastic materials in demanding applications has, however, been limited, among other things, by their deficient properties of strength and toleration of heat as well as by an unfavourable viscoelastic conduct, i.e. by a phenomenon of hysteresis. If the temperature of a thermosetting plastic is raised to a level higher than its glass transition temperature Tg, its mechanical properties collapse. If there are so-called micro cracks in the coating, in a dynamic situation the edges of these cracks may be rubbed against each other so that locally the Tg of the coating material is exceeded and a macroscopic damage arises in the coating. With thermosetting plastics, no effect of strengthening/toughening occurs when the temperature is raised to or beyond the glass transition range, but the properties of strength of a thermosetting plastic collapse in the glass transition range. This property is an essential Imitation of the permitted running conditions of a roll. Out of these reasons, rolls coated with thermosetting plastics cannot be operated at a temperature higher than 200xc2x0 C.
The need of polymer coatings for rolls which endure heat and have high strength has increased to a great extent in recent years. Owing to the limitations and difficulties related to thermosetting plastics, the possibility of finding new, alternative coatings among thermoplastic resins has also been examined, but a problem has been the lack of suitable materials and difficulties of manufacturing technology. Attempts have been made to solve these problems in the Finnish Patent Application No. 920501, in which a method has been suggested for coating a roll for a paper machine with a thermoplastic powder, together with a roll manufactured by means of the method. In such a case, the roll is coated by using a hypersonic plasma and a granular polymer powder in order to obtain the desired coating layer. This method is, however, expensive, the coating does not include any reinforcement material.
Thus, there is still a need to find a solution by whose means it is possible to provide a thermoplastic-coated roll whose properties and range of operation are more satisfactory and which is reasonable in respect of the manufacturing technique.
The objectives of the invention are achieved and the drawbacks of the prior art are overcome by means of the solution in accordance with the present invention.
The subject of the present invention is a thermoplastic-based roll coating (Shore D 70 . . . 98) for highly demanding applications, such as, for example, for supercalenders and soft calenders, which applications are ever more demanding in respect of the linear load, running speed, and temperature and of combinations of same in an attempt to provide a better quality level of paper. Further, the invention concerns a method for manufacture of a thermoplastic-coated roll, a composition of thermoplastic coating, a method of calendering by means of thermoplastic-coated rolls, as well as paper/board manufactured by means of thermoplastic-coated rolls.
The thermoplastic-coated roll in accordance with the invention, the method for manufacture of the roll, the composition of thermoplastic coating, the method of calendering by means of thermoplastic-coated rolls in accordance with the invention, and the paper/board manufactured by means of such rolls are characterized in what is stated in the patent claims.